Various improvements to battery stats collection

We now clear the battery stats when unplugging after the
battery is full.  This allows us to use the "total" stats as
a new "since last charged" stat.  Total is gone.  I never used
it, it was worthless.  Since last charged is a lot more
interesting.

The battery history now collects a lot more stats, and keeps
control over how much it can collect.  Printing is now more
descriptive.

The kinds of stats have been renamed to SINCE_UNPLUGGED and
SINCE_DISCHARGED.  The other two stats are still there, but
no longer printed; a future change will eliminate them
completely along with all of their state.

Change-Id: I4e9fcfcf8c30510092c76a8594f6021e9502fbc1